Manufacture of aromatic parahydroxyamines



Oct. 2, 1956 SPIEGLER 2,765,342

MANUFACTURE OF AROMATIC PARAHYDROXYAMINES Filed Oct. 22, 1952 2Sheeds-Sheet 1 Re ulafed ad ifion of 8 o nirrobenzene 2 4 6 8 IO l2 l4Tim e in hours %Complefion of reduction p Aminophenol Fiazg l l l I00200 300 400 500 600 700 Hydrogen parfiq! pressure-mm of Hg Rate ofHydrogenation l l l l l I l I00 200 300 400 500 600 700 Hydrogen partialpress re-mm ofHg INVENTOR LOUIS SPIEGLER BY 5', )WWALZZM ATTORNEY Oct,2, 1956 Filed 001:. 22, 1952 L. SPIEGLER 2,765,342

MANUFACTURE OF AROMATIC PARAHYDROXYAMINES FIG. 4.

2 Sheets-Sheet 2 Q Q) (D A g e n r El None (Contra/1 I Q Quaternaryammonium compoun 7's I X Non-ouafernory compounds I l I Y I l I 0.4 0.50.6 0.7 0.8 0.9 L0

Rafe of hydrogeno/I'on INVENTOR LOUIS 'SPIEGLER BY o/MWM ATTORNEY UnitedStates atent 6 MANUFACTURE OF AROMATIC PARAHYDROXYAMINES Louis Spiegler,Woodbury, N. 5., assignor to E. I. du Pont de Nemours and Company,Wiimington, Del., a corporation of Delaware Application October 22,1952, Serial No. 316,235

11 Claims. (Cl. 260575) This invention relates to an improved processfor the preparation of aromatic parahydroxyamines, and more particularlyto their preparation by the catalytic hydrogenation of nitro compounds.

A typical aromatic parahydroxyamine is p-aminophenol, which is avaluable intermediate in the manufacture of dyes and antioxidants. It iscommonly made by the reduction of p-nitrophenol obtained by nitratingphenol, or from p-nitrochlorobenzene. It is obvious that a potentiallymuch cheaper method involves the reduction of nitrobenzene tophenylhydroxylamine followed by rearrangement to p-aminophenol withoutisolation of the intermediate product. Such a process, in which thereduction of nitrobenzene is carried out with zinc dust, is described inU. S. Patent No. 2,132,454. The reduction may also be performedelectrolytically. In U. S. Patent No. 2,198,249 there is disclosed amethod by which mixtures of p-aminophenol and aniline are produceddirectly by the catalytic hydrogenation of nitrobenzene in mineral acidsolution, using a hydrogen pressure of 200 to 750 p. s. i. Yields ofp-aminophenol up to 57% are described in this patent. Although thisprocess may be operated successfully, the reaction does not proceed asrapidly as is desirable and requires the use of high pressure equipment.

It is an object of this invention to provide a process for thepreparation of aromatic parahydroxyamines by the catalytic hydrogenationof nitro compounds which proceeds rapidly, at low pressures and with theuse of relatively mild agitation, to give higher yields than haveheretofore been obtainable. A further object is to provide such aprocess in which the reaction may be controlled to give theparahydroxyamine and further reduction products in the proportionsdesired, e. g., in the reduction of nitrobenzene to producep-aminophenol and aniline in whatever proportions are desired. Furtherobjects will appear from the description which follows.

According to the present invention, an aromatic parahydroxyamine isprepared in admixture with the corresponding dehydroxylated amine by aprocess which comprises adding a nitro compound in which the nitro groupis attached to an aromatic nucleus which is unsubstituted in theposition para to the nitro group and hydrogen to a suspension of ahydrogenation catalyst in an aqueous solution containing from 1 to 25%by weight of sulfuric acid, at a temperature of from 50 to 145 C., thepartial pressure of hydrogen being maintained below 760 mm. of mercury,and the rate of addition of the nitro compound being such that at notime does the amount of unreacted nitro compound in the suspensionexceed its solubility therein.

It has been found that the process is further improved by having presentin the acid suspension at least 0.01%

Patented Oct. 2, 1956 by weight, based on the weight of dilute acidsolution, of a water-soluble quaternary ammonium compound.

In a representative and preferred embodiment of this processpara-aminophenol is made by the hydrogenation of nitrobenzene in thepresence of from 0.1 to 3% of a supported platinum catalyst (includingthe support) and from 0.01 to 0.2% of octadecyl trimethyl ammoniumchloride, in each case based on the weight of solution, at a temperatureof from to C. and a partial pressure of hydrogen below 500 mm. ofmercury, the rate of addition of nitrobenzene being as stated above.

The nitro compounds to which this invention is applicable are mononitroderivatives of aromatic compounds of the class consisting ofhydrocarbons containing no more than two benzene rings and monochloroand dichloro derivatives thereof. It is necessary that the position inthe aromatic nucleus which is para to the nitro group be unsubstituted,as this is the position to be occupied by the hydroxyl group. Inreferring to aromatic hydrocarbons containing no more than two benzenerings, it is intended to include compounds containing a single benzenering, compounds containing two unfused rings as in the compoundbiphenyl, and compounds containing two fused benzene rings as innaphthalene. Representative of this class of nitro compounds are:nitrobenzene, o-nitrotoluene, o-nitrochlorobenzene,m-nitrochlorobenzene, 2,5 dichloronitrobenzene, 2 nitrobiphenyl andl-nitronaphthalene.

An essential feature of this invention is the combination of theregulated addition of the nitro compound to the reaction vessel and theuse of subatmospheric partial pressure of hydrogen. In the process of U.S. 2,198,249 and in hydrogenations generally, the practice is to startwith the full charge of the compound to be reduced present in thereaction vessel and to carry out the reaction at high pressure. It hasnow been found, however, that by adding the nitro compound at such arate that no appreciable amount of it remains undissolved in thereaction mass, a very considerable increase in the rate of hydrogenationis obtained. The reaction proceeds smoothly and requires only mildagitation, whereas it has been found necessary to use high speedagitation in the process of U. S. 2,198,249. It is believed that afactor contributing to the improvement in the rate and smoothness of thereaction is the prevention of the formation of a separate phase ofnitrobenzene in the reaction mixture, which tends to coat the catalystand to decrease its activity.

Figure 1 in the accompanying drawing shows a graphical comparison of therate of hydrogenation which is obtained when nitrobenzene is addedslowly according to the process of this invention with the rate obtainedwhen all the nitrobenzene is added to the reaction kettle at the startof the reaction. The graph shows that under the conditions of this testreduction of the nitrobenzene is complete in six hours when the rate ofaddition is such as to avoid appreciable amounts of undissolvednitrobenzene, whereas fourteen hours are required when all thenitrobenzene is present in the reaction kettle from the beginning. Thereaction times shown in Figure 1 are illustrative but are strictlyaccurate only for the particular conditions of temperature, pressure,nature and amount of catalyst, acid concentration and degree ofagitation employed in this particular test. Under many conditions, thedifferences in reaction rates between the two processes are even morestriking.

A convenient method of adding the nitro compound 3 to the reactionmixture is by increments, each increment being added after itspredecessor is substantially completely reduced. In order to satisfy therequirement that no more unreacted nitro compound be present in thesuspension than is soluble therein, each incremental addition should beless than about 0.8% of the weight of dilute acid solution in thesuspension.

It is often desirable to add the nitro compound in the form of aconcentrated sulfuric acid solution. Commercial 96% acid is satisfactoryfor this purpose. Instead of adding the nitro compound in increments,satisfactory results are obtained by adding it continuously, providedthat the rate of addition is suitably slow. The entire reaction may alsobe carried out continuously by mixing streams of dilute acid containingthe catalyst, nitro compound either alone or in sulfuric acid solution,and hydrogen at the required rates and passing the mixture undersuitable agitation through a reaction tube or vessel.

The rate and character of the reaction are strongly influenced by thepartial pressure of hydrogen in the reaction vessel. The behavior ofnitrobenzene in this regard is representative. In this reaction, it hasbeen found that the ratio of p-aminophenol to aniline in the reactionproducts increases as the partial pressure of hydrogen decreases. Figure2 shows this effect graphically. This figure shows that, under theconditions of this test, in order to obtain a yield of p-aminophenolgreater than 80% with a corresponding yield of aniline of less than 20%,the hydrogen pressure should be about 250 mm. of mercury or less. Ifsubstantially pure p-aminophenol containing only small amounts ofaniline is desired as the immediate reaction product, even lower partialpressures of hydrogen may be employed. The graph also shows that as thepartial pressure of hydrogen approaches one atmosphere the reactionproduct becomes predominantly aniline. The actual relation between theamounts of paminophenol and aniline which are produced varies withchanges in the other reaction conditions such as temperature, amount ofcatalyst, and particularly the presence of a quaternary ammoniumcompound. In any event, operationof the present process at pressuresabove one atmosphere produces too small amounts of p-aminophenol to bepractical.

It will be seen that the superior results obtainable by the process ofthis invention depend on the combination of the use of low hydrogenpressures and slow addition of the nitro compound. Thus if nitrobenzeneis reduced using the slow addition technique but at the superatmosphericpressures of the prior art, the resulting product is almost entirelyaniline and various by-products such as ammonia and cyclohexane,practically no p-aminophenol being formed. On the other hand, use ofsubatmospheric hydrogen pressures without regulated addition of thenitro compound results in undesirably low rates of reduction.

As the partial pressure of hydrogen in the reaction vessel decreases,the over-allrate of reduction of the nitro compound also decreases. Thiseffect is shown graphically in Figure 3, where the rate of hydrogenationis expressed in terms of cubic feet of hydrogen consumed per hour. Thisefiect must be taken into account in selecting the optimum operatingconditions. Instead of using a very low hydrogen partial pressure inorder to obtain a high proportion of the parahydroxyamine in thereaction mixture, it is often preferable to use a somewhat higherpressure, thereby increasing the total amount of parahydroxyamineobtained in a given time. The optimum point will represent an economicbalance of factors such as the cost of separating the amines from theparahydroxyamines as compared with the cost of the reduction, theavailability of equipment, etc.

The partial pressure of hydrogen in the reaction vessel is equal to thetotal pressure in the vessel less the pressure of water vapor over theacid solution at the existing temperature. It is controlled by varyingeither the total pressure or the temperature. Partial pressures of 300to 500 mm. are convenient to maintain when the temperature is betweenand C., since the reaction vessel may then be kept under a totalpressure at or only slightly above atmospheric. Suitable reaction ratesand yields are obtained at these temperatures and pressures.

The reaction is carried out at a temperature of from 50 to 145 C. andpreferably at 75 to C. Below 50 C. the rate of reaction is impracticallyslow and above C. other reactions tend to occur. The rate of reaction isincreased with increasing temperature, whereas the proportion ofp-aminophenol in the reaction product appears to increase up to amaximum and then to decrease. At 500 to 540 mm. of hydrogen the yield ofpaminophenol from the reduction of nitrobenzene is a" maximum at 115 C.,while at 240 to 300 mm. the maxi mum yield is obtained at 100 C. Use oftemperatures in the range stated above as preferred results inconvenience of operation and in suitably high reaction rates withsatisfactory amounts of the desired parahydroxyamine in the reactionproduct.

The hydrogenation catalyst is preferably platinum supported on charcoalor other porous material. Other suitable hydrogenation catalysts includepalladium, rhodium, and the sulfides of certain heavy metals such asmolybdenum, cobalt and tungsten. The hydrogenation catalyst should beacid insensitive, i. e., it should not be rendered ineffective by thepresence of the sulfuric acid. The concentration of catalyst in thereaction mixture atfects the process in much the same way as does thepartial pressure of hydrogen. The rate of hydrogenation increases withincreasing catalyst concentration, but there is a substantialaccompanying decrease in the proportion of parahydroxyamine in thereaction products and also some increase in the amount of undesireddecomposition products. The optimum amount of catalyst depends on theother operating conditions and on the results desired. When the catalystconsists of 1% platinum supported on charcoal, its amount (including thesupport) is preferably from 0.1 to 3% of the weight of the dilute acidsolution.

The catalyst is suspended in a sulfuric acid solution containing from 1to 25% acid. If the acid is too dilute, incomplete transformation of theintermediate aryl hydroxylamine to parahydroxyamine is obtained.Particularly in the lower part of the operating range of temperature,both over-all reduction rate and yield of parahydroxyamine appear to befavored by acid concentrations of at least 10%. At high temperatures,the acid should be more dilute.

It has been found that quaternary ammonium compounds exert a profoundinfluence on the course of the hydrogenation, increasing the proportionof parahydroxyamine in the reaction product and at the same timeincreasing the rate of hydrogenation. The effect produced by thesecompounds is not merely a result of their dispersing action, since theimprovement is obtained with quaternary ammonium compounds which are notdispersing agents and is not obtained with other dispersing agents whichare not quaternaries.

The quaternary ammonium compound should be watersoluble and also shouldbe stable toward dilute sulfuric acid. into two separate ions (asubstituted ammonium cation and an anion such as a halide ion) or aninner salt such as a betaine. The compound preferably contains at leastone alkyl group containing at least ten and no more than eighteen carbonatoms, although compounds such as tetramethyl ammonium chloride are alsoeffective. Betaines having a normal alkyl radical of fourteen to sixteencarbon atoms substituted on the methylene carbon.

comprise another preferred group of compounds of this class.Representative quaternary ammonium compounds useful in the process ofthis invention include octadecyl trimethyl ammonium chloride, octadecyldimethyl. ethyl ammonium bromide, dioctadecyl dimethyl ammonium It maybe either of the kind which is ionizable.

chloride, dipentadecyl dimethyl ammonium chloride, dodecyl trimethylammonium chloride, C-cetyl betaine, C-dodecyl betaine and tetramethylammonium chloride. At least 0.01%, based on the weight of the diluteacid suspension, of the quaternary ammonium compound should be employedand ordinarily no further advantage is obtained by using more than 0.2%.

The mixture of reaction products which is present in solution at the endof the hydrogenation is separated by any convenient method. Thus themixture of p-aminophenol and aniline which is obtained from thehydrogenation of nitrobenzene may be separated as described in U. S.Patent No. 2,198,249, by filtering off the catalyst, adding alkali toliberate the amines from their salts, removing the aniline by steamdistillation and allowing the p-aminophenol to crystallize on coolingfrom the aqueous solution, after concentrating and adjusting to pH 6.0to 6.3 if necessary. Alternatively, the sulfuric acid may be neutralizedwith lime, the aniline steam-distilled olf, the calcium sulfate removedfrom the hot solution by filtration and the p-aminophenol recovered as apowder from the aqueous solution by spray drying.

In the illustrative examples which follow, Examples 1 and 2 describehydrogenations of nitrobenzene carried out at the same temperature butat different hydrogen partial pressures, and show the effect of thisvariation upon the rate of hydrogenation and the ratio of p-aminophenolto aniline. Example 3 shows the effect of lower temperature and higherpartial pressure of hydrogen. Examples 4 and 5 illustrate the effect ofdecreasing the acid concentration. The efiect of decreasing the amountof catalyst is shown in Example 6. Example 7 describes a preferred setof conditions chosen to give a practical balance between yield andreaction rate. In Example 8 the efiect of various quaternary ammoniumcompounds and of non-quaternary dispersing agents are compared. Thehydrogenation of nitro compounds other than nitrobenzene is shown inExample 9.

Example 1 A 10 gallon glass-lined reaction kettle equipped with apropeller agitator is charged with:

2,250 g. of 96% sulfuric acid 20,500 g. of copper-free water 10.5 g. ofC-cetyl betaine 1.75 g. of metallic platinum dispersed on 175 g. ofcharcoal The kettle is flushed with hydrogen, and then heated to 100 GilC. while sweeping through a vent with a slow stream of hydrogen. Thekettle is then closed, and the pressure is built up to 200 mm. of Hgabove the atmospheric pressure with hydrogen gas. Nitrobenzene is thenpumped in in 25 gram portions, while hydrogen gas is fed simultaneouslyas rapidly as reduction occurs. The pressure is maintained at 200 mm.:mm. (gage). Pressure changes on a mercury-filled manometer are observedto determine when to add nitrobenzene and how to adjust the hydrogenflow. As long as the pressure tends to fall, indicating the presence ofunreduced nitrobenzene, the i'eed of hydrogen is continued so as to keepthe pressure substantially constant. When absorption stops, anotherincrement of nitrobenzene is introduced. The actual average partialpressure of hydrogen in the reaction vessel averages about 240 mm. Inthis manner, 2500 g. of nitrobenzene are reduced in less than 5 hoursgiving a rate of 560 g. of nitrobenzene per hour.

The analyzed yield is 83.2% of p-aminophenol, and 16.8% of aniline,based on the nitrobenzene.

Example 2 Nitrobenzene is reacted by the procedure described in Example1 except that the temperature is held at 98il C. and the pressure ismaintained at 0 to 20 mm. (gage) with hydrogen, resulting in an averagepartial pressure of hydrogen of about mm. in the reaction vessel. Underthese conditions, 1250 g. of nitrobenzene are reduced in 9 hours. Theyield of p-aminophenol is 98.6% and of aniline 1.3%, in each case basedon the amount of nitrobenzene reduced.

Example 3 Nitrobenzene is reacted by the procedure described in Example1 except that the temperature is held at 65 +1 C. and the pressure ismaintained at 0 to 20 mm. (gage) with hydrogen, resulting in an averagepartial pressure of hydrogen of about 600 mm. in the reaction vessel.Under these conditions, 2500 g. of nitrobenzene are reduced in 6 hours.The yield of p-aminophenol is 27.2%and of aniline 72.4%.

Example 4 The equipment used in the preceding examples is charged with:

19,480 g. of copper-free water 1.75 g. of platinum dispersed on 17 5 g.of activated carbon 1070 g. of 96% sulfuric acid Operating as in Example1, but at 500 mm. gage pressure and 100 C., resulting in a hydrogenpartial pressure of about 540 min., and feeding a solution consisting of1845 g. of nitrobenzene and 767 g. of 96% sulfuric acid in 25 g.portions, 1845 g. of nitrobenzene are reduced in 4 hours. The analyzedyield in the reduction liquors is 47.3% of p-aminophenol and 52.7% ofani line, based on the nitrobenzene.

Example 5 The equipment used in the preceding examples is charged with:

19,300 g. of water 214 g. of 96% sulfuric acid 1.75 g. of platinumdispersed on g. of activated carbon Operation under the same conditionsof temperature, pressure and method of nitrobenzene addition as inExample 4 results in a 22.9% yield of p-aminophenol and 77.1% yield ofaniline. The rate of reduction is 420 g. of nitrobenzene per hour.

Example 6 The equipment used in the preceding examples is charged with:

18,450 g. of water 2150 g. of 96% sulfuric acid 0.35 g. of platinumdispersed on 35 g. of activated carbon The temperature is maintained at100 C. and the pressure at 500 mm. gage, giving a hydrogen partialpressure of about 540 mm. A solution of nitrobenzene in concentrated 96%sulfuric acid containing 2 moles nitrobenzene per mole of sulfuric acidis added in 25 g. portions. The rate of reduction is 330 g. ofnitrobenzene per hour, and the yield in the reduction liquors is 65.3%of paminophenol and 34.7% of aniline, based on the nitrobenzene.

Operating as above but with 0.17 g. of platinum, the rate of reductionis 193 g. of nitrobenzene per hour, with yields of 70% of p-aminophenoland 30% of aniline.

Example 7 Nitrobenzene is hydrogenated in a 5 liter creased flask havingfour vertical creases to act as baffles and equipped with a stirrerhaving a vertical 5.5 inch blade, a circular lower edge and a maximumheight of 1.5 inches. The stirrer is operated at 700 to 800 R. P. M. Theflask is provided with a heating jacket, inlet and outlet tubes forhydrogen, a manometer and an inlet for the solution of nitro compound.Into the flask are placed 1000 g. of water, 66.2 g. of 96% sulfuricacid, 0.75 g. of C- cetyl betaine and 0.016 g. of a platinum catalystsupported on 1.6 g. of charcoal. A mixture of 68.2 g. of nitrobenzeneand 28.2 g. of 96% sulfuric acid is added to the flask at a rate ofbetween 0.5 and 0.6 cc. per minute per liter of solution, while at thesame time hydrogen is added at such rate that the pressure in the flaskremains essentially constant. The temperature is maintained at 8788 C.and the total pressure at 760 mm., the partial pressure of hydrogenbeing about 300 mm. The sulfuric acid added with the nitrobenzene isequivalent to the basic reduction products formed from the nitrobenzene,so that addition of the mixture to the flask produces no change in theoverall acidity. Hydrogen is absorbed at a rate of between 0.70 and 0.42cu. ft. per hour. The nitrobenzene is added in about 120 minutes. Thesolution then contains 52.4 g. of p-aminophenol and 7.0 g. of aniline,equivalent to yields of 87% and 13% respectively.

Example 8 Nitrobenzene is hydrogenated as in Example 7, using thequaternary amonium compounds and dispersing agents shown below. Thereaction temperature in each case is 78 C. and the total pressure oneatmosphere, giving a partial pressure of hydrogen of 460 mm. The amountof platinum catalyst used is 0.02 g., supported on 2 g. of charcoal.Between 0.5 and 0.7 g. of the quaternary or other agent is added. Therate of reduction shown below is expressed as cc. of nitrobenzenehydrogenated per minute per liter of charge. The results of thesereductions are as follows:

Rate of Yield of Agent reduction p-aminophenol Percent None (control) 0.525 55 Quaternary compounds:

O-eetyl betaine 0.830 78 O-dodecyl betaine 0.672 73 Dodecyl triruethylammonium chloride- 0. 837 77 Octadecyl trimethyl ammonium chloride.. 1.005 80 Octadecyl dimethyl ethyl ammonium bromide 0. 570 80 Dioctadecyldimethyl ammonium chloride 0. 500 80 Dipentadecyl dimethyl ammoniumchlori e 0. 564 80 Tetramethyl ammonium chloride 0. 084 64Non-quaternary compounds:

Triethylamine sulfate 0. 483 57 Tributylamine sulfate 0. 472 58Dioctadscyl propyleneamine dioleate 0. 404 68 Ethylcne-diaminetetraacetic acid 0. 70 55 Condensation product from ethylene oxide andoleic alcohol 040 71 Example 9 Each of the nitro compounds shown in thetable below is hydrogenated by a procedure similar to that described inthe two preceding examples. Octadecyl trimethyl ammonium chloride isused in each case. The nitro compound, fused when necessary, is addedwithout sulfuric acid (other than that present initially in the flask)except where noted. The hydrogenation of o-nitrotoluene is. carried outat 78 C. and 460 mm. partial pressure.

of hydrogen, while all of the other compounds listed are hydrogenated atC. and 300 mm.

Catalyst Nitro compound cone, g. Products Formed Yield,

Pt/1,000 cc. Percent solution 3-methyl-4-amino- 80. 1 o-nitrotoluene 0.0215 phenol.

' o-toluidine 19. 9 l-nitronaph thalene 0. 0215 {iggfigggg%g figtfi" g3-chloro-4-amino- 81. 4 o-nitrochlorobenzenc 0. 0200 phenol.

o-chloroaniline 18. 1 3-chloro-4-amino- 86. 4 0. 0100 phenol.

o-chloroaniline l3. 6 IllPbkCillOfOXllttO- 2-chloro-4-ami11o- 32. 0

benzene 0. 0200 phenol.

m-chloroaniline 68. 0 2,5-dlchloro-4- 26. 02,5-diohloronitroaminophenol.

benzene 0. 0200 o-and m-chlorol9. 2

1 aniline.

2,5-dichloroaniline 54. 8 2-aInil1o-5-hydroxy- 73. 2 2-nitrobiph enyl 0.0200 biphenyl.

2-an1inobipheny1 26. 8

1 Added as a 33% solution in 96% H S 0 In Examples 1-6 above, includingthose in which no dispersing agent is present, there is used only amoderate amount of agitation, furnished by a propeller stirrer at 300 R.P. M., while in Examples 7-9 a flat bladed stirrer operating at 700800R. P. M. is employed. For larger quantities in larger equipment, therequirements are still easily met by the type of agitation ordinarilyavailable. On the other hand, the hydrogenation described in U. S.2,198,249 requires speeds of 2500 R. P. M. and peripheral velocities of1500 ft; per minute for best results. By the process of the presentinvention, the ratio of paminophenol to aniline may be widely varied atwill and may be increased until p-aminophenol is almost the onlyproduct. Compared again with the process of, U. S. 2,198,249, theprocess of the present invention operates faster, at lower temperaturesand pressures and with smaller amounts of catalyst, and producessubstantially 100% combined yield of p-aminophenol and aniline. Forthese reasons, it lends itself readily to continuous operation asdescribed above.

This application is a continuation-in-part of my U. S. applicationSerial No. 259,988, filed December 5, 1951.

I claim:

1. A process of preparing an aromatic parahydroxyamine by the reductionof a nitro compound which is a mononitro derivative of an aromaticcompound of the class consisting of hydrocarbons free from non-aromatichydrocarbon substituents other than lower alkyl radicals and containingno more than two benzene rings and monochloro and dichloro derivativesthereof, said nitro compound having its nitro group attached to anaromatic nucleus which is unsubstituted in the position para to thenitro group, which comprises adding the said nitro compound and hydrogento a suspension of an acid-insensitive hydrogenation catalyst in anaqueous solution containing from 1 to 25% by Weight of sulfuric acid, ata temperature of from 50 to C., the partial pressure of hydrogen beingmaintained below 760 mm. of mercury and the rate of addition of thenitro compound being such that at no time does the amount of unreactednitro compound in the suspension exceed its solubility therein.

2. A process according to claim 1 in which the acid suspension containsfrom 0.01% to 0.2% by weight, based on the weight of dilute acidsolution, of a water-soluble quaternary ammonium compound of the groupconsisting of tetraalkyl ammonium halides containing at least 1 and notmore than 2 alkyl groups of at least 10 carbon atoms and not more than18 carbon atoms, and betaines containing a C-normal-alkyl group of from12 to 16 carbon atoms.

3. A, process of preparing para-aminophe'nol which comprises addingnitrobenzene and hydrogen to a suspension of an acid-insensitivehydrogenation catalyst in an aqueous solution containing from 1 to 25%by weight of sulfuric acid, at a temperature of from 50 to 145 C., thepartial pressure of hydrogen being maintained below 760 mm. of mercuryand the rate of addition of nitrobenzene being such that at no time doesthe amount of unreacted nitrobenzene in the suspension exceed itssolubility therein.

4. A process according to claim 3 in which the acid suspension containsfrom 0.01% to 0.2% by weight, based on the Weight of dilute acidsolution, of a water-soluble quaternary ammonium compound of the groupconsisting of tetraalkyl ammonium halides containing at least 1 and notmore than 2 alkyl groups of at least carbon atoms and not more than 18carbon atoms, and betaines containing a C-normal-alkyl group of from 12to 16 carbon atoms.

5. A process according to claim 4 in which the quaternary ammoniumcompound is octadecyl trimethyl ammonium chloride.

6. A process according to claim 4 in which the quaternary ammoniumcompound is a betaine having a normal alkyl radical of fourteen tosixteen carbon atoms substituted on its methylene carbon.

7. A process according to claim 4 in which the quaternary ammoniumcompound is C-cetyl betaine.

8. A process according to claim 3 in which the nitrobenzene is added inthe form of a concentrated sulfuric acid solution.

9. A process according to claim 3 in which the hydrogenation catalyst isa supported platinum catalyst.

10. A process according to claim 3 in which the nitrobenzene is addedincrementally, each increment being less than 0.8% of the weight ofdilute acid solution in the suspension and being added after thepreceding increment is substantially completely reduced.

11. A process of preparing para-aminophenol which comprises addingnitrobenzene and hydrogen to a suspension of a supported platinumcatalyst in an aqueous solution containing from 1 to 25% by Weight ofsulfuric acid and from 0.01 to 0.2% by weight of octadecyl trimethylammonium chloride, at a temperature of from to C., the amount ofcatalyst (including its support) being from 0.1 to 3% of the weight ofsolution, the partial pressure of hydrogen being maintained below 500mm. of mercury and the rate of addition of nitrobenzene being such thatat no time does the amount of unreacted nitrobenzene in the suspensionexceed its solubility therein.

References Cited in the file of this patent UNITED STATES PATENTS2,198,249 Henke et a1 Apr. 23, 1940 2,292,879 Kise Aug. 11, 19422,587,572 Tyron Feb. 26, 1952 2,619,503 Benner et al Nov. 25, 1952FOREIGN PATENTS 559,730 France Sept. 30, 1923

1. A PROCESS OF PREPARING AN AROMATIC PARAHYDROXYAMINE BY THE REDUCTIONOF A NITRO COMPOUND WHICH IS A MONONITRO DERIVATIVE OF AN AROMATICCOMPOUND OF THE CLASS CONSISTING OF HYDROCARBONS FREE FROM NON-AROMATICHYDROCARBON SUBSTITUENTS OTHER THAN LOWER ALKYL RADICALS AND CONTAININGNO MORE THAN TWO BENZENE RINGS AND MONOCHLORO AND DICHLORO DERIVATIVESTHEREOF, SAID NITRO COMPOUND HAVING ITS NITRO GROUP ATTACHED TO ANAROMATIC NUCLEUS WHICH UNSUBSTITUTED IN THE POSITION PARA TO THE NITROGROUP, WHICH COMPRISES ADDING THE SAID NITRO COMPOUND AND HYDROGEN TO ASUSPENSION OF AN ACID-INSENSITIVE HYDROGENATION CATALYST IN AN AQUEOUSSOLUTION CONTAINING FROM 1 TO 25% BY WEIGHT OF SULFURIC ACID, AT ATEMPERATURE OF FROM 50 TO 145* C., THE PARTIAL PRESSURE OF HYDROGENBEING MAINTAINED BELOW 760 MM. OF MERCURY AND THE RATE OF ADDITION OFTHE NITRO COMPOUND BEING SUCH THAT AT NO TIME DOES THE AMOUNT OFUNREACTED NITRO COMPOUND IN THE SUSPENSION EXCEED ITS SOLUBILITYTHEREIN.